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September 2, 2009

September 2009 Geology and GSA Today media highlights

by Sam Savage

GEOLOGY includes studies of the Fraser River delta, British Columbia; the impact of global climate change on microfossil communities; alluvial fans in Taiwan; earthquake ruptures; earth-flows along the Eel River; Mediterranean fossil whales; collecting bias and carnivorous dinosaurs of the Kem Kem Formation, Morocco; and the effects Hurricanes Cindy, Katrina, and Rita on barrier island systems. GSA TODAY tells a 20-million-year-old story of interactions among the Columbia River, volcanic eruptions, glacial floods, and tectonic events.

Highlights are provided below.

Mazzotti et al. present results about ongoing natural and man-made subsidence in the Fraser River delta region, greater Vancouver, Canada, and its impact on future relative sea-level rise. A combination of land-based and satellite geodetic data is used to map the variations in subsidence rate across various geological and anthropogenic settings of the Fraser River delta. From this data set, it is concluded that (1) slow natural subsidence of the delta is mainly due to natural consolidation of the young and shallow part of the sediment column, and (2) the loading from large man-made structures and buildings can be responsible for an increase in ongoing subsidence, by a factor of 2-5 times. Thus, in the Fraser River delta, as in most deltaic, lacustrine, and alluvial environments, man-made structures can result in increased subsidence and significant amplification of future relative sea-level rise.

Global climate change has often resulted in species extinctions, but the impact on communities of organisms is more difficult to measure. Webb et al. use a commonly applied ecological tool, rank-abundance curves, to evaluate the response of deep-sea microfossil communities to the Paleocene-Eocene thermal maximum (PETM), which may be considered as an analog for current and future global warming. The two types of microfossil communities studied showed distinctly different responses. One type of community (benthic foraminifera) became increasingly stressed during the PETM, with the community response occurring before the recognized extinction event. In contrast, the other type of community (ostracodes) became less stressed during the same interval, reinforcing the idea that different groups of organisms respond differently to extinction events and global warming. The decoupling of community response from extinctions during the PETM reaffirms that future climate change could have far-reaching effects on species and ecosystems, and proves the importance of examining the responses of both species and communities during extinction events.

Usually, the slope of a landform such as a watershed and an alluvial fan decreases with its size. However, Lin et al. have found that relatively large watersheds and alluvial fans in Taiwan have nearly constant slopes regardless of their sizes. V-shaped valleys in the watersheds subject to rapid uplift and erosion tend to take a threshold slope angle. Sediment and water supply from such watersheds subject to typhoon-induced heavy rainfall have common characteristics, leading to similar depositional slopes.

In recent years, scientists have used geological records of subtle changes in the chemistry of ancient precipitation to estimate the geological history of elevation changes in mountain ranges like the Himalaya, the Andes, and the Sierra Nevada. These studies were based on the idea that precipitation falling on mountains should become more depleted in heavy isotopes of hydrogen and oxygen as the mountains rise. The study by Galewsky uses a computer model of airflow over topography to show that the basic assumption of these geological studies may not be valid. Galewsky shows that precipitation isotope ratios may not always decrease as mountains rise, and that the isotope ratios may change in response to climate change rather than changes in mountain elevation. Studies of elevation changes in mountain ranges are important for improving our understanding of plate tectonics, but this new study suggests that existing interpretations may need to be modified to include changing climate and changes in topography unrelated to elevation.

Some cracks near seismic faults in Earth open almost instantaneously, due to the passing of earthquake rupture fronts moving at thousands of meters per second. Aspects of such cracks, such as their appearance, orientation, and the spacing between them, depend on the physics of the earthquake rupture front and the velocity at which the front is moving. Therefore deeper understanding of how these cracks form provides us with an indirect way to learn about the physics of earthquake rupture. Griffith et al. use optical experiments and high-speed photography to interpret the origins of arrays of opening cracks that form during dynamic shear ruptures in Homalite-100, a transparent material used as an analogue for rocks. The geometry of these crack arrays varies predictably with the velocity at which the rupture front moves. Results of this study represent an important potential bridge between geological observations of structures preserved along exhumed faults and theoretical models of earthquake propagation, potentially leading to diagnostic criteria for interpreting velocity, directivity, and stress state associated with past earthquakes on exhumed faults, and providing geologists with a new tool to use ancient faults to better understand modern day earthquake physics.

Earth-flows are in important erosional process in many landscapes and can deliver significant amounts of sediment to stream networks, but relatively little is known about how earth-flows evolve or vary beyond the decadal scale. To better understand longer-term earth-flow evolution, Mackey et al. studied the behavior of an active earth-flow in the Eel River catchment, northern California, USA. By combining traditional surveying, analysis of displaced trees on sequential aerial photos, inventories of cosmogenic nuclides in soil pits, and high-resolution LiDAR-derived topography, they documented about 150 years of earth-flow activity, including estimates of pre-historic earth-flow velocities. Notable was an acceleration of earthflow movement up to 4 m per year during the 1960s, probably due to rainfall, followed by a dramatic slowing over the past 30 years as the supply of new earthflow material has become exhausted. They found that rates of localized earthflow erosion exceed 10 m per year, more than 20 times the background erosion rate, and argue that active earthflows are transient but vigorous erosional features in the landscape, exhibiting periods of movement interspersed with long periods of inactivity.

Thrasher and Sloan used a regional climate model to examine, in high resolution, the sensitivity of the early Eocene (about 50-56 million years ago) climate to different levels of carbon dioxide in the atmosphere. Because this model can resolve topography at high resolution, climate simulations of the mountainous region of western North America are far superior to those simulations created by a global climate model. This provides a more realistic evaluation of how atmospheric carbon dioxide affects climate on a regional scale, gives a greater perspective on the true nature of how carbon dioxide forced the early Eocene climate of western North America, and improves our understanding of how climates have changed in the past and how our current climate may change in the future.

Sunken carcasses of large whales can be a habitat for organisms specialized in the extraction of energy from organic compounds without the aid of oxygen. Some clams and mussels exploiting energy from whale bone lipids have close evolutionary connections with organisms living in deep marine hot springs, and are at the same time distantly related to shallow marine bivalves adapted to aerobically exploit other resources. The fossil record has proved useful to understand evolutionary connections in geological time. New insights are now available thanks to the discovery of an exceptionally complete and articulated, 3-million-year-old Mediterranean whale and its associated shelled mollusks. The study of this fossil association, and of other Mediterranean fossil whales sunken at shallower depths, by Dominici et al., and its comparison with the deep sea record, show that reducing habitats from intermediate depths (100-150 meters) are particularly fit for the introduction of evolutionary novelties. Such habitats are, in fact, close enough to the coast to benefit from the genetic diversity of source populations, but distant enough to experience lower stress from competitors for space and food.

Detailed investigation by Brand et al. of material (carbonate) from an ancient seamount in Japan shows that open oceans and epeiric seas are chemically linked for strontium isotopes, questionable for oxygen isotopes, and different with respect to carbon isotope compositions. This has major implications when identifying global events (e.g., mass extinctions) using the latter two isotopes, as well as for global chemostratigraphic correlations of sedimentary units, in that their veracity is seriously challenged by the findings of this study.

Uba et al. shed light on the controversy surrounding the Miocene South American marine incursion from Colombia through Peru and Bolivia into Argentina. They present, for the first time, a well-constrained depositional age of the Yecua unit in the central sub-Andean region, thereby enhancing the better correlation of the Miocene marine strata in South America. In general, this paper contributes to the crucial topic of discrimination between marine and freshwater depositional environments through integration of multidisciplinary studies. Their data document four short-lived probable marine ingressions between 12.4 and 8 million years ago in the Bolivian Subandes.

Global warming is causing rapid acceleration of the melting of the Greenland ice sheet. That meltwater is destined for the North Atlantic, where it is expected to interact with branches of ocean circulation that currently have a warming effect on the North Atlantic region. Daley et al.'s findings show that, on the last occasion when large quantities of meltwater from a decaying ice sheet were discharged through the Labrador Sea, the northeastern North American seaboard was strongly affected, most likely leading to plummeting temperatures within a few decades. The ensuing complex cold event was much more severe than is currently predicted by state-of-the-art numerical models. Their findings exemplify the link between changes in the ocean and its impact over land.

One of the five largest mass extinctions in Earth's history occurred at the Triassic-Jurassic boundary, and this article by Williford et al. reports evidence for major changes in the sulfur cycle discovered in marine sediments deposited during and after the Triassic-Jurassic mass extinction. They suggest that the changes in sulfur cycling resulted from a combination of global effects, such as the opening of the Atlantic Ocean, as well as local effects such as increased erosion rate that may have occurred as a result of land plant extinctions. The largest perturbation in the sulfur cycle coincides with similar, previously reported changes in the carbon cycle. Carbon and sulfur are both biologically important elements and their biogeochemical cycles act as critical links between the biosphere, lithosphere, ocean, and atmosphere. This study is further evidence that major mass extinctions tend to be associated with disruptions in these cycles.

Recent hurricane seasons drew renewed attention to hurricanes and associated storm surges. Research on the geological imprints of storm surges can provide insight into the hydrodynamic conditions during a storm surge and the history of a coastline. In the study by Spiske and Jaffe, a coarse-grained hurricane sequence from the island of Bonaire (Netherlands Antilles) is described and hydrodynamically interpreted. The ridge sequence, consisting of coral rubble, was deposited during Hurricane Lenny in November 1999. Vertical textural variations and sedimentary structures are caused by time-dependent hydrodynamic changes. Changing angles of imbrication reflect alternating flow directions and changes in velocity. Normal grading during setup and inverse grading during return flow are caused by decelerating and accelerating flow, respectively. These parameters were used to subdivide the sequence into depositional units that correspond to different stages of the surge, such as setup, peak, and return flow. For each of these stages, hydrodynamically controlled sedimentation processes and transport modes were inferred. Additionally, this study also gives criteria for distinguishing between hurricane-and tsunami-generated coarse clast deposits.

McGowan and Dyke investigate whether the reported high number of carnivorous dinosaurs from the Kem Kem Formation of Morocco might be influenced by the large amounts of material recovered by commercial collectors. The other possibility is that the proportion of predatory taxa in the Kem Kem was much higher than other well-known and well-studied Late Cretaceous terrestrial ecosystems. Using data collected in the field, and a survey of fossil material available for sale in fossil shops in Marrakech, McGowan and Dyke used statistical methods to test whether the fossil shops and field collection were sampling the major vertebrate groups in similar proportions. Their initial results indicated that this was not the case, so a computer simulation was written to generate thousands of simulated samples that were then compared to the actual proportions of the major groups. Using this simulation, they found that the proportions of taxa between the field and shop collections were different. In the shop collections, the remains of turtles and their relatives were underrepresented, while dinosaur material was somewhat overrepresented. A number of possible explanations are possible for why so many carnivorous dinosaurs are known from the Kem Kem. McGowan and Dyke's findings establish that collecting bias is a credible mechanism of accounting for at least a portion of the unusually high number of carnivorous dinosaurs from this area.

An isolated Early Cretaceous limestone lens from the Crimean Peninsula, containing masses of articulated specimens of the rhynchonellide brachiopod Peregrinella, formed at a hydrocarbon seep. Although a relation of such deposits with high-abundance but low-diversity fossil assemblages to hydrocarbon seepage was previously established, no criteria existed to constrain the intensity of seepage. Similarly, an assessment of the composition of seepage fluids was only straightforward if extreme carbon isotope patterns were observed. But many ancient seep carbonates show only moderate depletion in the heavy stable carbon isotope (13C), based on which it is impossible to discriminate seepage of methane from seepage of crude oil. Using molecular fossils (i.e., lipid biomarkers), Peckmann et al. reconstruct the composition of seepage fluids and the intensity of past seepage for a deposit for which such a classification was previously not possible. Comparison of biomarker patterns with cement abundance in the Hauterivian and other seep limestones indicates that the new criteria can be taken as a measure for seepage intensity. This finding has great potential for the study of ancient seep environments, to better constrain past rates of emission of the potent greenhouse gas methane.

The landfall of Hurricanes Cindy, Katrina, and Rita along the Louisiana coast in 2005 provided an opportunity to document the effects of large-magnitude storms on barrier island systems. The impact of short-term events is particularly relevant to Louisiana, where long-term historical analyses have demonstrated rapid degradation of barriers and a large deficit in the coastal sediment budget. Miner et al. are the first to document storm impacts on ebb-tidal deltas - the sandy shoals that form landward of tidal inlets - by comparing pre-and post-hurricane season seafloor surveys to quantify sediment erosion and accretion. Their results show that the seafloor fronting the barriers underwent widespread erosion as a result of the storms. Using wave numerical models to estimate sediment transport during the storms, they demonstrate that the sand component is retained within the regional coastal system, whereas the mud is exported. Large volumes of mineral sediment are deposited on marshes during storms, helping them accrete vertically and keep pace with rising seas; however, the origin of this sediment has not been identified. Miner et al. explain that muddy sediment eroded from offshore is transferred landward to the interior bays and marshes during storm events by high-velocity, landward-directed currents that are associated with surge inundation as storms make landfall. This study emphasizes the role of frequent and intense hurricanes as a mechanism for regional landward sediment transfer during transgression.

Tejada et al. describe original high-resolution osmium-and carbon-isotopic records of the organic-rich Cretaceous marine sedimentary sequence from Central Italy, including the Selli Level type section, formed during the Early Aptian oceanic anoxic event (OAE1a). They highlight the link between large volcanic eruptions, abrupt environmental changes, and unusual biological events. They report, for the first time, the seawater osmium-isotope profile across the Selli Level interval formed 124-122 million years ago, when global anoxia occurred in the oceans. The data provide the first direct evidence that the main phase of the Ontong Java Plateau eruption coincided with OAE1a, suggesting that OAE1a was triggered by massive volcanism, and providing a time frame for plateau emplacement. The data show two negative shifts. The first shift suggests an early phase of volcanism that predates OAE1a event by about 300,000 years. The second shift corresponds to the peak period of eruption, (about 500,000 to 1 million years in duration), indicating that the bulk of the plateau was emplaced within about 1-2 million years. The protracted period of negative osmium-isotope shifts during the OAE1a event coincides with increasing 13C-12C, suggesting contemporaneous enhanced deposition of organic matter. The results are consistent with the combined 40Ar-39Ar and 187Re-187Os ages for Ontong Java Plateau lavas, which suggest that most of the plateau was emplaced between 119 and 126 million years ago, but do not have the resolution to reveal any details of the eruption history.

The 2000 km long, continental-scale Columbia River is one of North America's greatest rivers. Sourced deep in the Southern Rocky Mountain Trench of southeastern British Columbia, the river crosses the Cordilleran orogen, entering the Pacific Ocean at Astoria, Oregon, USA. Almost unique among Earth's great rivers, the Columbia cuts a deep gorge through an active volcanic arc - the Cascadia arc. In the September issue of GSA Today, Russell Evarts of the U.S. Geological Survey and his colleagues trace the river's passage through the arc, showing how the river flowed through and was captured by the rapidly subsiding Portland Basin. Sedimentary and volcanic strata preserved in the basin, a structural depression within the Cascadia arc, tell a 20-million-year story of interaction between the river and the surrounding volcanoes. Based on recent geological mapping, LiDAR and geophysical surveys, and paleomagnetic and geochronological data, Evarts et al. tell a story rich not only in local volcanic, tectonic, and climatic events, but one that includes major continental scale events, including damming of the river by massive flows of basalt and inundation by colossal glacier outburst floods.